feat(socks5): add udp.resolveDomain option for clients incompatible with ATYP=Domain

Add a  wrapper that intercepts WriteTo on the
client-facing SOCKS5 UDP relay and resolves domain addresses to IPs
before the SOCKS5 header is encoded. Controlled via the
 (or ) handler metadata flag.

When enabled, SOCKS5 UDP response datagrams always carry ATYP=IPv4 or
ATYP=IPv6 — never ATYP=Domain (0x03) — making them compatible with
clients like tun2proxy and Surge that cannot parse domain-typed
addresses in UDP.

The wrapper resolves domains through the router's infrastructure:
host mapper → resolver → system DNS fallback. Unresolvable domains
cause the datagram to be silently dropped rather than forwarded in a
format the client rejects.

Fixes go-gost/gost#434

handler/socks/v5/metadata.go

@@ -26,6 +26,7 @@ type metadata struct {
 	enableBind        bool
 	enableUDP         bool
 	udpBufferSize     int
+	udpResolveDomain  bool
 	compatibilityMode bool
 	hash              string
 	muxCfg            *mux.Config
@@ -58,6 +59,7 @@ func (h *socks5Handler) parseMetadata(md mdata.Metadata) (err error) {
 	h.md.enableBind = mdutil.GetBool(md, "bind")
 	h.md.enableUDP = mdutil.GetBool(md, "udp")
 	h.md.udpBufferSize = mdutil.GetInt(md, "udp.bufferSize", "udpBufferSize")
+	h.md.udpResolveDomain = mdutil.GetBool(md, "udp.resolveDomain", "udpResolveDomain")
 
 	h.md.compatibilityMode = mdutil.GetBool(md, "comp")
 	h.md.hash = mdutil.GetString(md, "hash")